WASHINGTON — Over the years, Sen. Dianne Feinstein has come to view herself as a China hand. And now, amid mounting tensions between China and Taiwan, the California Democrat is increasingly asserting herself behind the scenes as a would-be peacemaker. In the process, Feinstein is playing on a rare set of personal relationships that she has nurtured with the three men now at the center of the standoff: the presidents of China, Taiwan and the United States. To each, Feinstein has been dispensing strong and unequivocal advice. "I sometimes say that in my last life maybe I was Chinese," Feinstein...